I think the brother might be just an airhead....or he is a manipulative guy.
Well, it is similar to my own life...I am the brother by the way.
I am academically gifted, cheerful, easy to make friends and to my country standard, I am quite good looking. So my little sister is often compared to me in which saying I am better.
Even though a lot of times my sister was also praised for her own talent, I think the frequency of being compared as the lesser one is higher.
The main thing is "DO NOT COMPARE YOUR CHILD WITH ANYONE" doesn't matter if it is a good or a bad comparison. Just say she/he is bad/good and what is the standard of good you expect...I think that is better.
Example of her attitude:
1. She (7 yo) was playing tag with her friends. Then I (10yo) came to play too. I didn't mean anything bad, I just want to play with my little sister and her friends. But then she cried loudly and shouted at me "Whenever you are around, my friends don't want to play with me! I hate you!" It shocked me....so I remember, even if my little sister (now 24) doesn't.
2. I studied seriously in high school and got a scholarship for a program that let me finish bachelor degree and directly go to master in a much shorter time than usual AND got me a secure well paying job. She was not happy...she sulked and said something like "why are you so lucky? It's not like you study...you always play games and all that. I study seriously, why am I still stupid!" and give me a complicated glare. Like a little hateful a little sad a little helpless.
And confused me can only said, "What are you talking about? I study too, just not at home." LOL great move there brother! sigh..
3. I am talking about my friends and the silly things happened at work. She laughed (not polite laugh, the tearful stomach hurts one) and asked questions. Then I asked about hers. She suddenly becomes unenthusiastic with even saying "why do you wanna know about my friends?", "shut up", "none of your business." what the heck...
Back to the manga...Since the trauma is only projected from her perspective, I hope the brother is not an ei-hol (I am not sure if this site censor bad words in comments lol)
Insecurities and trauma are really hard to deal with. hopefully, you guys can manage your mental health.
In case you are wondering why I am still with my sister even though we are young adults, I live in a country where it is normal to live with your parents if you are not yet married. While living in your own house/apartment, not your parents, just because you are an adult is seen as unfilial.